When a flange portion is formed by press forming a metal sheet between tools of press forming, stretch deformation (a stretch flange) may occur by a bent end portion of the flange portion of the metal sheet receiving a tensile force. Such forming is called “stretch flange forming”. In stretch flange forming, if the stretch deformation exceeds the deformation limit of the metal sheet, a crack is generated. This crack is called “stretch flange crack”. In particular, a stretch flange crack easily occurs in a formed part of a high-strength steel sheet, for example, a press formed part for an automobile. If a stretch flange crack is generated, a prescribed part shape may not be obtained.
As a method of avoiding such a stretch flange crack, for example, in Patent Literature 1, a method of suppressing generation of a stretch flange crack by improving a state of an end face of a part where a crack tends to be generated is disclosed. Further, in Patent Literature 2 and Non-Patent Literature 1, a method of giving excess metal by tools of press forming is described. Further, in Patent Literature 3 and Patent Literature 4, a method of using a blank shape in which a stretch flange crack is hard to be generated is disclosed. Further, in Non-Patent Literature 2 and Non-Patent Literature 3, a method of distributing deformation, suppressing centralization of the deformation at a stretch flange part, and avoiding generation of a stretch flange crack, by implementing forming using a sequential contacting punch is disclosed.